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Motor tuning and Limits in series
« on: September 21, 2008, 02:22:10 AM »
Hello all,
I have some general doubts in mach config and motor tuning.
Regarding the motor tuning,on what factor one shld decide the velocity and acceleration settings.
Steps per unit can be calculated my considering the steps of motor(full or half) and ball screw pitch.but how abt vel and acceleration.
iam driving my motor(at half stepping) at 12v @1A with 1mm pitch lead screw.(motor is rated at 5V@1A)

and i already asked below question at CNCzone.but didn't get any reply.so iam just posting it here.
Quote
i agree using the limits as home switches(as mach is capable of doing that)
but what abt wiring the limits in series(which i found most of the users doing).
say iam jogging towards X+ and touched that side limit.now iam supposed to jog towards X-(using limit override function).here how the mach know which side iam jogging.as the limits are in over ride condition and we wired them in series mach dosen't know which side is to jog and which side to stop.
i tried this on my machine.the axis touched the X+ limit,so i clicked limit override and again jogged toward X+ and it crossed the limit.
so how safe it is.(am i doing something wrong here)
i was planning for another parallelport,so that i can assign separate input for each limit switch.imean for X+ and X-
so wiring the limits in series,how fool proof it is

Regards

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Re: Motor tuning and Limits in series
« Reply #1 on: September 21, 2008, 02:53:18 AM »
Hello all,
I have some general doubts in mach config and motor tuning.
Regarding the motor tuning,on what factor one shld decide the velocity and acceleration settings.
Steps per unit can be calculated my considering the steps of motor(full or half) and ball screw pitch.but how abt vel and acceleration.
iam driving my motor(at half stepping) at 12v @1A with 1mm pitch lead screw.(motor is rated at 5V@1A)

and i already asked below question at CNCzone.but didn't get any reply.so iam just posting it here.
Quote
i agree using the limits as home switches(as mach is capable of doing that)
but what abt wiring the limits in series(which i found most of the users doing).
say iam jogging towards X+ and touched that side limit.now iam supposed to jog towards X-(using limit override function).here how the mach know which side iam jogging.as the limits are in over ride condition and we wired them in series mach dosen't know which side is to jog and which side to stop.
i tried this on my machine.the axis touched the X+ limit,so i clicked limit override and again jogged toward X+ and it crossed the limit.
so how safe it is.(am i doing something wrong here)
i was planning for another parallelport,so that i can assign separate input for each limit switch.imean for X+ and X-
so wiring the limits in series,how fool proof it is

Regards

The simple answer to your jog on limit question is you have to Jog the right way :) Mach will jog off a limit at a reduced rate and will only go at full rapid again after you stop and restart jogging for this very reason. If you have seperate switches it will make no difference, it can still be jogged the wrong way. However if you then set up the softlimits Mach will stop just before it hits a physical limit and will prevent you jogging onto the physical limit, you will then be able to jog only away from the limit, think this is what you are looking for :)

As for the motor tuning, well that is entirely hardware dependant. No one will be able to tell you what you will be able to set for acceleration or velocity unless their machine is exactly the same as yours in every respect. Even the computer could make a difference as some people might be able to get a max reliable  kernel speed of 25KHz and the next persons computer may be very happy at 100KHz. Afraid the velocity and accel is just trial and error.

Hood
Re: Motor tuning and Limits in series
« Reply #2 on: September 21, 2008, 04:09:15 AM »
Thanks for the reply hood.
I'll do that trail and error method for motor tuning.

and regarding the jogging issue.i thought,that if i place 2 limits for each axis,and  i click Limit over ride,it won't let me jog in the wrong direction.but it seems if i use limit over ride condition,it will completely disable the  limit switches.and the system will let me jog any direction.which is much more dangerous for me.
so softlimits is the only thing which can save me here i guess.then how can i configure them.imean how to setup soft MAX and MIN.on what basis.
(actually iam making small demo machines for students,which i want to make completely fool proof.say by mistake if they jog in the wrong direction,the axis will come out  :-\ )

Regards

Offline Hood

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Re: Motor tuning and Limits in series
« Reply #3 on: September 21, 2008, 04:59:33 AM »
Limit Override wont disable the limits, it will still stop if a limit is hit, you then have to press the reset button and you can then again jog. As you have said it is possible to jog the wrong way. I can see why you would want to make this foolproof but maybe that is what is wrong with the world nowadays, people try to make things as safe as possible so that  no harm can come to people or things and that makes people not think about their actions which in turn leads to more danger when things dont work as expected ;D

 Softlimits are exactly what you are looking for, they will stop the axis moving beyond the soft limit in either direction. To set them up depends on where you have your home switches positioned, if you have them so that the tool  is lower left corner of the table (looking from above) and the tool is furthest away from the table in the Z axis then X and Y Min and Z Max is set to zero, then X Max, Y Max  and Z Min are set to each axis travel. The Z will be a negative number but X and Y will be positive.
 If you have your home switches at any other positions then you will have to set a home off value dependant on their position.

Hood
Re: Motor tuning and Limits in series
« Reply #4 on: September 21, 2008, 09:55:57 AM »
thanks hood.
so only a single limit switch will be enough to home and limit the axes.
i guess ppl are using one limit switch along with the home switch just for more safety.
so how shld i place my limit and home switches.imean home switch towards X+ and limit switch towards X-.or both of the at one side only(then which comes first).

Offline Hood

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Re: Motor tuning and Limits in series
« Reply #5 on: September 21, 2008, 12:09:48 PM »
Usually you have one limit that is placed approx mid travel of the axis, you can then trigger this from either end of the travel. That way you only need one switch for each axis and you can then asign the way you want to move when using it as a home switch.
 There is no added benefit of using a seperate switch as far as safety is concerned. If you want to position a home switch other than at the end of an axis travel then you will need a seperate switch for home but thats about the only reason normally.
 I use seperate limit switches on the lathe and  new mill as Mach doesnt do the homing, I leave that up to my servo drives as they look for the switch then back off and look for the index pulse of the motors encoder, this makes it deadly accurate.
Hood
Re: Motor tuning and Limits in series
« Reply #6 on: September 21, 2008, 02:10:25 PM »
As safety is main criteria here for me and by keeping ur suggestions in my mind,i think 
i'll place 2 limits at the extreme ends and will use one of them as home switch.so using soft limits,i''ll limit the travel.worst case scenario if the soft limits is disables the other extreme limit switch will stop the axis.
and for using one switch as both home and limit,i'll connect NO contact of limit switch to home input and NC contact to limit input.
http://img261.imageshack.us/my.php?image=newbitmapimagenp4.gif

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Re: Motor tuning and Limits in series
« Reply #7 on: September 21, 2008, 02:50:04 PM »
Yes you can do it that way but there is no real need. Mach ignores limits when homng and treats them as home switches. Once homing is complete then they become limit switches again.
 Yes best to have limits at either end of the axis but as I said one switch can be used for that if wanted.

Hood